The skull of Acanthostega was more derived than those
of rhipidistian fish but retained features from its fish ancestry.
The braincase of Acanthostega was similar to that of fish although the
occipital region had begun to fuse in the region of the notochord (Clack,
Compared to earlier osteolepiforms, Acanthostega's snout was longer, its
eyes were located farther back on its head and more medially, and the
otic region was shorter. Acanthostega lost its lateral rostral bone around
the nostril. Much of the region around the nostril was composed of cartilage
rather than bone, perhaps a juvenile trait retained from its larval stage.
Its choana was larger but similar to that of sarcopterygian fish. The
hyomandibula was modified to become the stapes. Bones around the external
nostril were lost as fish adapted to smelling in air. Acanthostega retained
the lateral line system in the skull which had served its fish ancestors
as a sensory system for hunting aquatic prey. Although the shoulder girdle
had separated from the skull to create a neck, Acanthostega still possessed
an anocleithrum like fish (the anocleithrum is one of the bones which
had attached the shoulder to the skull in fish; Clack, 2002). The following
illustration depicts the homology of the shoulder girdles of primtive
fish and early tetrapods and contrasts the fish which possessed bones
(in blue and red) to attach the shoulder to the skull.